Many types of missiles use a variety of aerodynamic surfaces for lift, control and stability. Depending on the manner in which a missile is stowed or launched, it is often necessary to make some or all of the surfaces foldable or retractable to reduce the overall size of the missile.
Some types of aerodynamic surfaces are in the form of fins which fold against the missile body and may be curved to fit closely around the body. Since these usually curve in the same direction to fit in the available space, they do not provide symmetrical lift when erected and are suitable only for directional stability. Such folding fins are often not flush with the body and can cause considerable drag at high speeds.
Other types fold or retract into the body and occupy internal space, which restricts the space available for payload. For high speed flight the surfaces need not be very large and some compromises are acceptable. For low speed flight, on the order of 200 to 300 ft/sec, the surface area must be fairly large to be effective and this poses problems of stowage. Flexible wings have been used, in which a membrane is supported by a spar of strut which swings out from the body. The single layer of fabric normally used, while stretched out by its supporting member, is subject to aerodynamic flutter at certain speeds and airflow conditions. When used as a lifting wing the fabric bows upwardly to form an undercambered single surface airfoil, which is reasonably stable under consistent loads. However, sudden changes in load conditions can cause the wing to collapse or flutter.
It is desirable, therefore, to have a wing which will fold into a small space in the manner of a flexible wing and has simple support structure, yet which will resist fluttering and maintain aerodynamic stability under varying load and airflow conditions.